1. Field of the Invention
The present invention relates to a laser beam scanning optical system.
2. Description of the Prior Art
A type of laser beam scanning optical system is known that forms images by using a plurality of laser beams having different wavelengths. Such a laser beam scanning optical system is used, for example, to form color images on silver-halide film by using laser beams having wavelengths corresponding to red, green, and blue colors. When a plurality of laser beams having different wavelengths are dealt with by a single scanning lens system, it is inevitable to take chromatic aberration into consideration.
Chromatic aberration includes longitudinal chromatic aberration and lateral chromatic aberration. Longitudinal chromatic aberration causes laser beams of different wavelengths to be focused at different positions along a line parallel to the optical axis. However, since laser beams exhibit relatively large depths of focus, longitudinal chromatic aberration does not cause serious problems. On the other hand, lateral chromatic aberration causes laser beams of different wavelengths to be focused at different positions along a line perpendicular to the optical axis (i.e., in the main scanning direction). As a result, laser beams of different wavelengths scan different scan widths on the image plane, and this degrades the quality of produced images.
As a scanning lens system that corrects such lateral chromatic aberration, Japanese Laid-open Patent Application No. S62-262812, for example, proposes an f.theta. lens. This f.theta. lens is composed of optical elements made of carefully selected optical materials to achieve correction of lateral chromatic aberration.
However, in conventional laser beam scanning optical systems, sufficient correction of lateral chromatic aberration is possible only by the use of a combination of a concave lens and a convex lens, each having a strong power. Inconveniently, this not only makes it difficult to obtain satisfactory speed uniformity and image-plane flatness, but also requires a greater number of lens elements as well as extremely high production accuracy.